Moons of Mars
Phobos and Deimos: the tiny moons of Mars
Mars has two small, dark, potato-shaped moons named after the sons of the Greek war god Ares: Phobos (fear) and Deimos (dread). Both were discovered by Asaph Hall in August 1877.
NASA/JPL-Caltech/University of Arizona, MRO HiRISE (PIA10368), 23 Mar 2008. Public domain.
Meet the moons
Phobos
- Size
- about 27 × 22 × 18 km
- Distance above Mars
- about 6,000 km
- One orbit
- 7 h 39 min
Phobos orbits so close and so fast that it goes around Mars three times a day. Seen from Mars it rises in the west and sets in the east, twice a day.
Its biggest crater, Stickney, is 9 km wide, almost half the moon's width. The impact nearly shattered it. Long grooves run across the surface.
Phobos is doomed. Mars's tides are pulling it inwards by about 1.8 m every century. In roughly 30 to 50 million years it will either crash into Mars or be torn apart into a ring.
Deimos
- Size
- about 15 × 12 × 11 km
- Distance above Mars
- about 23,500 km
- One orbit
- 30.3 hours
Deimos is smaller, smoother and farther out. Its craters are softened by a thick blanket of dust. From the surface of Mars it looks like a bright star.
Unlike Phobos, Deimos is very slowly drifting away from Mars, just as our Moon drifts away from Earth.
Where did they come from?
Their dark colour looks like certain asteroids, so for a long time scientists thought Mars captured two passing asteroids. But their neat, nearly circular orbits above Mars's equator are hard to explain that way. Another idea is that, like our Moon, they formed from debris thrown up by a giant impact on Mars.
Japan's MMX (Martian Moons eXploration) spacecraft launched on 20 October 2026. It is planned to reach Mars in 2027, land on Phobos, scoop up a sample and bring it to Earth around 2031. The sample should settle the question.
Could life be here?
Our rough guide, from 0 (no chance) to 5 (among the best places to look). No life has been found beyond Earth.
Could life be here? Phobos and Deimos
- Some water may be bound in minerals
- No atmosphere, no liquid water
- Too small to keep any heat
Verdict: No. But Phobos may hold dust blasted off Mars, a record of the Red Planet's past.
Quick answers
Why does Phobos rise in the west?
Phobos orbits Mars faster than Mars spins, so it overtakes the planet's rotation and appears to rise in the west and set in the east.
Will Phobos crash into Mars?
Yes, eventually. Tides are pulling it inwards, and in roughly 30 to 50 million years it will hit Mars or break up into a ring.
Who discovered the moons of Mars?
American astronomer Asaph Hall, in August 1877, at the US Naval Observatory.
Sources and credits
- NASA Science, Mars moons.
- JAXA, MMX launch result, 20 October 2026.
- NASA/JPL/University of Arizona, HiRISE images of Phobos and Deimos.
Photos credited to NASA, NOAA and other public-domain sources are used with thanks; BotBuhdy adds a small botbuhdy.com mark. Pictures labelled AI-generated illustration are artist-style illustrations made for this page, not photographs.